The present invention is concerned with a component that functions with surface acoustic waves. Components that function with surface acoustic waves are known hereafter as SAW components. Filters based on such components are used, for example, in mobile radio devices or in reception paths of data communication systems.
An exemplary SAW component has at least one acoustic track arranged on a piezoelectric substrate, which generally contains at least one electroacoustic transducer with a preferably periodic interdigital electrode structure for exciting a surface acoustic wave, whose wavelength λ corresponds approximately to the period of the electrode structure. The device may have two reflectors surrounding the transducer for localizing the acoustic wave in the transducer region. The surface acoustic wave excited on piezoelectric substrates, for example a Rayleigh wave, a shear wave or a longitudinal wave, has an acoustic component and an electrical component. The acoustic component of the wave is a mechanical deflection relevant to the used wave type of the material on the substrate surface. The mechanical deflection causes a corresponding electrical potential in the piezoelectric material. Thus, the surface acoustic wave takes along an electric wave component which is usually in phase with it.
The electrode structures represent mostly electrode fingers or electrode strips which are arranged interdigitally alongside one another, which have electric excitation centers if the electrode fingers arranged alongside one another of a finger pair are at a different potential. The electric excitation centers are such places in the transducer at which locally excited electric components of the electroacoustic wave going in the forward direction and one going in the backward direction are in phase together. The excitation centers are usually in the center of an electrode finger and possibly in the center of a split electrode finger.
Due to electrical and mechanical discontinuities, on each electrode finger, a part of the incident acoustic wave is reflected in the backward direction. It is assumed that the reflection is localized at a point or reflection center, at which the reflection coefficients of waves going in opposite directions are the same or rather purely imaginary. On most piezoelectric substrates which have symmetrical direction-independent characteristics in terms of reflections, this is the center of the finger.
The interdigital transducers in a SAW component are mostly bidirectional. This means that they radiate a surface acoustic wave in both longitudinal directions, for example, without a predominant direction.
For example, transducers with equidistantly arranged electrode strips of the same width, which are preferably λ/4, are known and are hereafter called normal finger transducers. Moreover, split finger transducers are known in which an electrode strip at a certain potential is split to form a split finger. In a simply split finger, for example a two-part, split finger, the electric excitation center is approximately in the center of the split.
Moreover, unidirectional interdigital transducers are also known in which, due to a special arrangement of reflectively acting electrode fingers compared to exciting electrode finger pairs, the radiation of the acoustic wave is obtained preferably in one direction, the amplitude of the wave going in a preferred direction or forward direction being significantly greater than the amplitude of the wave going in the opposite direction or backward direction.
It is possible to build unidirectional transducers on isotropic piezoelectric substrates and it is possible to achieve a preferred radiation direction of a transducer with, for example, three or four electrode fingers per wavelength. Moreover, single-phase unidirectional transducers, commonly called SPUD Ts, are known, which makes it possible to obtain the unidirectionality of the transducer with only one electrode pair per wavelength if, for example, different electrode finger widths are used. The unidirectional radiation of the acoustic wave arises in that a wave excited in the backward direction and reflected at a discontinuity point in the vicinity of the corresponding excitation center, in the forward direction, is constructively superimposed with the wave excited at the same excitation center going in the forward direction. This is achieved in a SPUDT through individual reflecting strips which are arranged in the vicinity of the exciting electrode fingers. Since the distance between the exciting and the reflecting electrode fingers is comparably large in a SPUDT, only a small bandwidth of the component can be realized. This disadvantage can be circumvented through the configuration of an interdigital transducer or converter with a period varying in the transverse direction of the electrode fingers. Here, the electrode fingers and the finger period diminish and the associated wavelength decreases, so that the measured-in-wavelengths finger center distances and the finger widths (independent of the transversal coordinates) remain constant. U.S. Pat. No. 4,973,875, whose disclosure is incorporated herein by reference thereto, and GB 2 212 685, which claims priority from the same two Japanese Patent Applications as DE 38 38 923, disclose interdigital transducers called fan transducers, such as illustrated in FIGS. 1A and 1B, by the interdigital transducers W1, W2 and W1′ and W2′. A fan transducer can be divided in the transverse direction into subtracks which are discretely or continuously passing over one another and are lined up on one another, so that, in each subtrack, an acoustic wave with a specified wavelength is being excited.
Some radio broadcasters use both terrestrial and also satellite-based signal transmission for broadcasting their programs. Here, the satellite signal can be received only if a clear line of sight exists from the receiver antenna to the satellite. Otherwise, a terrestrial base station, which acts as a repeater, is used to bridge the shadowing. An exemplary frequency allocation for different signal types is shown schematically in FIG. 2A. The signal which is transmitted terrestrially is transmitted or rather received at a frequency which lies between the frequencies used for satellite-based signal transmissions.
A superheterodyne receiver shown in an exemplary manner in FIG. 2B has, in each case, a separate signal path for receiving the satellite signal (signal path SP) and the terrestrially transmitted signal (signal path TP). The received signal is mixed down to a center frequency or in steps using mixers M1–M4 in each case to a lower frequency, for example 315 MHz and subsequently to 75 MHz in Sirus Radio. After each mixer, a bandpass filter B1–B4 is provided. Broadband SAW filters, for example fan transducer arrangements, are particularly well suited for this application.
Due to strong power level differences between terrestrial and satellite-based signals, undesired interference to the two signal types can occur, under certain circumstances, in the receiver. In order to decrease this effect, the range around the center frequency of the filter should be suppressed by using notched filters. Here, the transfer range for satellite-based signals should ideally not be disturbed.
It is possible in a fan transducer to suppress the signal transmission, which is caused through excitation of an acoustic wave, at certain fingers in the passband by reducing the overlap length of the electrode fingers of the subtrack formed for excitation of the acoustic wave at this frequency. This solution has the disadvantage that for long transducers, the angle of the terminal fingers deviates highly from the normal to the wave propagation direction, it is also true that the increase in the fan angle in the corresponding subtrack would lead to an overlapping of the adjacent fingers. For the shortened overall length of the transducer, the excitation strength of the transducer is decreased.
Moreover, it is possible to apply a dampening mass on the subtrack, which excites the signals to be suppressed. However, this requires a precision which cannot be implemented currently with previously known techniques, such as screen printing.
Another possibility consists of arranging dampening structures or structures for diverting the acoustic wave between the input and output transducer, but this is difficult to implement due to the limited chip length.
Moreover, there exists the possibility to replace the transducers in each case with two subtransducers in order to suppress the corresponding frequency band in the center of the passband range. If the subtransducers are connected in parallel, there arises additional connecting structure which cannot always be connected directly to further components arranged on the same substrate of the filter circuit and which increases the electromagnetic crosstalk of the parts of the circuit. A series connection of the subtransducers has the disadvantage that here the impedance of the arrangement is quadrupled.